1. Field of the Invention
This invention relates to an image display apparatus adapted to utilize electron beams such as a field emission display (FED).
2. Related Background Art
Research efforts have been and being paid for developing large image display apparatus utilizing a Braun tube (CRT) or some other device having an image displaying effect in order to meet the large demand for such displays. Large display apparatuses are by turn required to be thin and light weight. Additionally, they have to be manufactured at low cost. However, the CRT is designed to accelerate electrons by a high voltage and then deflect accelerated electrons in order to excite the fluorescent substance laid on a face plate. Therefore, the CRT theoretically has a significant length and hence it is difficult to obtain a thin and lightweight CRT. The inventors of the present invention have been engaged in the development of surface conduction electron-emitting devices and image display apparatus comprising surface conduction electron-emitting devices.
For example, the inventors have tried to apply a multi-electron-beam source as shown in FIG. 9 of the accompanying drawings. FIG. 9 is a perspective view of an image display apparatus realized by using a multi-electron-beam source.
Referring to FIG. 9, the image display apparatus comprises a cathode ray tube formed by arranging surface conduction electron-emitting devices 4001, row-directional wirings 4002 and column-directional wirings 4003, of which the row-directional wirings 4002 and the column-directional wirings 4003 are so disposed as to produce a passive matrix. The display additionally comprises an outer container bottom 4004 (which may also be referred to as rear plate) carrying the multi-electron-beam source 4002, a side wall 4005 (which may also be referred to as support frame or outer container frame) and a face plate 4006 having a fluorescent layer 4007 and a metal back 4008. The fluorescent layer 4007 of the face plate 4006 includes phosphors that are excited by electron beams to emit light and a black matrix adapted to suppress reflections of external light and prevent the different colors of the phosphors from mixing. A high voltage is applied to the fluorescent layer 4007 and the metal back 4008 by a high voltage source 4011. Thus, the fluorescent layer 4007 and the metal back 4008 operate as anode.
Appropriate electric signals are applied to the row-directional wirings 4002 and the column-directional wiring 4003 of the multi-electron-beam source having a passive matrix wiring arrangement in order to drive selected ones of the surface conduction electron-emitting devices so as to output electron beams in an intended way. For example, to drive the surface conduction electron-emitting devices of a row of the matrix, a selection voltage Vs is applied to the row-directional wiring 4002 of the selected row and non-selection voltage Vns is applied to the row-directional wirings 4002 of all the unselected rows. In synchronism with the above voltage applications, a drive voltage Ve is applied to the column-directional wiring 4003 in order to cause them to output electron beams. With this technique, a voltage of Ve-Vs is applied to the surface conduction electron-emitting devices of the selected row and a voltage of Ve-Vns is applied to the surface conduction electron-emitting devices of the unselected rows. Therefore, the devices of the selected row can be made to output respective electron beams with different intensities by selecting appropriate values for the voltages Ve, Vs and Vns and differentiating the drive voltages Ve that are applied to the respective column-directional wirings 4003. Since surface conduction electron-emitting device shows a high response speed, the time length during which a surface conduction electron-emitting device outputs an electron beam can be changed by changing the duration of application of the drive voltage Ve.
The electron beams output from the multi-electron-beam source 4001 as a result of application of voltages as described above then irradiate the metal back 4008, to which a high voltage Va is being applied, to excite some or all of the phosphors arranged there as targets. As a result, the phosphors that are irradiated with an electron beam emit light. Thus, the above described arrangement operates as image display apparatus when voltage signals are applied thereto as a function of a given piece of image information.
In short, as a high voltage (which may also be referred to as acceleration voltage or anode voltage) is applied to the metal back 4008 that is part of the anode electrode of an image display apparatus having the above described configuration in order to generate an electric field between the rear plate 4004 and the face plate 4006 and accelerate the electrons emitted from the electron beam source 4001, which by turn excite the phosphors and cause them to emit light, an image is formed on the display apparatus. Since the luminance of the image display apparatus heavily depends on the acceleration voltage, a high acceleration voltage is required in order to raise the luminance of the displayed image. On the other hand, in order to realize a thin image display apparatus, the thickness of the image display panel of the image display apparatus needs to be reduced. Then, the distance separating the rear plate 4004 and the face plate 4006 needs to be made very small. As a result, a considerably strong electric field is produced between the rear plate 4004 and the face plate 4006.
However, a display panel of the above described type is accompanied by the following problems.
FIG. 10 of the accompanying drawings is a schematic cross sectional view of the display panel of an image display apparatus of the type under consideration. The image display apparatus comprises a rear plate 2005 having an electron beam source 2002 and a face plate 2007 having an anode 2104 and an acceleration voltage Va is being applied to the anode 2104. Note that the anode 2104 is electrically insulated by the vacuum gap separating the face plate 2007 and the rear plate 2005 and the surfaces of the face plate 2007 and the rear plate 2005. The dimensions of the vacuum gap define the depth of the image display panel, while the length and the width of the surface of the face plate 2007 and those of the surface of the rear plate 2005 define the area and the width of the region of the image display panel that is not used for displaying an image. Therefore, it is highly desirable that all these dimensions show a small value. However, as these dimensions are reduced, the display shows large electric field strength if compared with a display whose corresponding dimensions are not so small when the same voltage is applied to the anode 2104. Then, the former display shows an increased electric discharge probability. An electric discharge can remarkably degrade the image quality of the images produced by the image display apparatus and hence is a serious problem particularly when the reliability of image display apparatus is to be improved.
Particularly, since the rear plate 2005 and the face plate 2007 are generally glass-made members and the electric insulation of the surface of a dielectric plate such as a glass plate is much poorer than that of a vacuum gap, it is very important to improve the withstand voltage of the surfaces of those plates that are made of glass.
Meanwhile, there are known image display apparatus comprising an electric potential defining electrode 2106 formed on the surface of the rear plate 2005 or the face plate 2007 where the anode 2104 is arranged as shown in FIG. 11. The electric potential defining electrode 2106 is arranged there in order to define the distribution of electric potential on that surface and limit the region that is subjected to an electric field. The electric potential of the electric potential defining electrode 2106 is lower than that of the anode 2104. For example, EP 1117124 discloses an image display apparatus comprising such an electrode. If there is a structure located outside the image region of an image display apparatus and subjected to an electric field (in other words, located in a space subjected to an electric field), the electric field can be concentrated there depending on the profile of the structure to eventually give rise to an electric discharge. This is the reason why such an electric potential defining electrode 2106 is formed there. The electric potential defining electrode 2106 is designed to define an electric potential lower than that of the anode so as to alleviate the intensity of the electric field existing outside of itself.
There is also known a technique of arranging a high voltage supply terminal 2107 on the rear plate 2005 as shown in FIG. 12 in order to feed the anode 2104 on the face plate 2007 with electricity. Since the electron beam source 2002 arranged on the rear plate 2005 accelerates electrons, the potential difference between the electron beam source 2002 and the anode can become very large. Then, there can arise a problem of electric discharge between the high voltage supply terminal 2107 and the electrode 2018 that is closest to the high voltage supply terminal 2107 among the electrodes located on the rear plate 2005.
The arrangement of an electrode arranged on the surface of the member where the region defined by the anode is located and having an electric potential lower than the electric potential of the anode gives rise to the following problems.
Firstly, if an electrode to which a high voltage is applied has a complex profile that may includes a projection, generally the electric field is concentrated there to consequently give rise to an electric discharge. Secondly, as an electric discharge takes place, the electrode can be destroyed by the discharge current and become no longer electrically conductive if partly. Then, there arises a part where the electric potential is not defined. Techniques that can be used to prevent the electrode from producing a complex surface profile include the use of a thin film process for preparing the electrode. Specific examples of such techniques include vacuum evaporation and sputtering. Electrodes prepared by means of such techniques are generally relatively thin. A thin electrode can easily be destroyed by electric discharge. On the other hand, if an electrode is prepared by using a thick film that is formed by way of a thin film process in order to prevent the electrode from being destroyed, the stress in the film can be raised during the thin film process. A thick film process such as a screen printing process may alternatively be used for preparing an electrode. However, an electrode prepared by using such a technique can have a coarse surface that shows undulations, which by turn can give rise to an electric discharge. Techniques for coating the insulating surface arranged between the electrode showing an electric potential that is defined to be equal to that of the anode and the electrode showing an electric potential that is defined to be low are also being developed. However, when the electrode showing a low electric potential is prepared by using a thick film process along with such a technique, there are occasions where the high resistance film does not connect the low potential electrode well due to the following reason. While it is preferable to prepare the high resistance film by using a thin film that is made as thin as possible from the viewpoint of reducing the power consumption rate, the low potential electrode requires a certain thickness so that it may satisfactorily define an electric potential. Then, the thickness of the high resistance film and that of the low potential electrode show a large difference to consequently give rise to a problem (defective coverage) in the region where the high resistance film covers the low potential electrode. Such a defective connection can also give rise to an electric discharge and hence improvements have been required to the technique of using a high resistance film.